1. Sensation & Perception

2. Questions How do we see right side up when the image is upside down?
What causes illusions?

3. How do we see right side up?
The images on our retinas are upside down and reversed left-to-right.
This is due to the focusing of light by the corneas and lenses in the eyes.
Somehow our brains figure out how to correctly perceive the environment.

4. Misconception Alert! The eyes do not send beams into the environment!
Vision works because light enters the eyes.

5. How do we see right side up?
The brain does not have to flip the images.
The brain deals only with neural signals.
The sense organs convert energy from the environment into neural signals through the process called transduction.

6. What causes illusions?
The sensory information coming from the sense organs (bottom-up) is usually not sufficient for accurate perception.
The brain must use its knowledge (top-down) to strategically interpret the sensory information.
These top-down strategies cause both accurate perception and illusions.

7. What causes illusions? Example 1
The Moon Illusion
Lack of depth information in the retinal images
Perceived size depends on both size of the image and perceived distance of the object.

8. What causes illusions? Example 2
Müller-Lyer Illusion
Importance of context

9. What causes illusions? Example 2
Why don’t we notice the blind spots in our eyes?
Axons of ganglion cells form the optic nerve.
Where the optic nerve exits from each retina, no photoreceptors exist; this is the blind spot.
Use of context

10. What causes illusions? Example 3
How are colors perceived?
Trichromatic Theory
Opponent Process Theory

11. What causes illusions? Example 3
Negative color afterimages
Sensory adaptation